Modern Step-scan Interferometry

www.spec t roscopyon l i ne . com T he concept of step-scan mid-infrared inter ferometr y dates back to the 1960s when the first optoelectronically controlled step-scan interferometer was reported (1). In contrast to the linear change of the optical path difference (retardation) in a conventional continuous-scan interferometer, the retardation changes stepwise in a step -scan Fourier transform–infrared (FT-IR) spectrometer. Step -scan FT-IR spectroscopy eliminates the time dependence of the interferogram and temporal Fourier frequencies created by a continuous scan. This makes step-scan FT-IR spectroscopy a very powerful tool for studying timeand phase-dependent phenomena (2). However, because of the growing acceptance of commercial continuous-scan instruments and the lack of exploration of step -scan applications, continuous-scan FT-IR spectroscopy was the only commercial product available for many years before serious commercial step -scan interferometer development took place (3–5). Research interests in photothermal– photoacoustic IR spectroscopy and dynamic time-resolved spectroscopy in the late 1980s led to development activities of step-scan instruments both academically (6) and commercially (3). However, the use of a step-scan interferometer was initially limited to only a few research laboratories because the earlier step-scan inter ferometers were ver y sensitive to environmental vibrations and had relatively poor mirror-position control. Stepscan FT-IR spectroscopy has increased in popularity among researchers in both academia and industry since the early 1990s. The driving force behind this popularity is the extensive pioneering work performed in advanced dynamic spectroscopic applications. The commercialization of a step-scan spectrometer with a robust, digital signal processor (DSP); precise mirror-position control; and turnkey operation has made step-scan FT-IR the technical choice for a variety of applications in chemistry, physics, materials, and biomedical fields (10–12).